Scandium, although quite abundant, is relatively under-utilised in industry as there are no convenient (high grade) sources of the metals. Important low level sources of scandium include uranium tailings and the waste sludge of titanium, zirconium and red mud. The sludge from such plants consist largely of mixed iron, manganese, titanium, tungsten and other hydrous oxides with varying levels of scandium, typically in the range of 10 to 100 ppm.
Processes which attempt to recover scandium from these materials face the problem of the waste stream being extremely complex and heterogeneous chemically. About two dozen other elements are present in greater or lesser amounts. Furthermore, the low level of scandium present, mandates processing large quantities of the sludge. Patented procedures for the separation of scandium from iron and other metals have disadvantages which make them ill-suited to the large scale production of scandium. For instance, an extraction-based procedure (U.S. Pat. No. 5,019,362) involves adjusting the pH of an acidic solution to a terminal value of 3.0 to 3.5. The solution is filtered. The acidic solution is mixed with a chelating resin in the hydrogen form, in an ion exchange column and again adjusting the pH to a terminal value between 1.9 and 2.1. The resin is rinsed with a mineral acid and the scandium is removed from the resin by passing diglycolic acid through the column. However, during the pH adjustment, metal hydroxides precipitate (especially if titanium is present in solution) causing scandium to precipitate as well. Up to 40-50% scandium can be lost during this step.
Another method (see RU patent 2,062,810) is carried out by sorption of scandium from hydrochloric solution with the help of N-(2-oxipropyl)-N′-(20oxi-3-pyridinium propyl)-N″-methylene phosphonium polyethylene polyamine followed by washing and desorption. The washing takes place with water, and desorption is carried out using a carbonate or fluoride solution. While this method achieves separation of scandium from iron and other metals, the selectivity of the amphoteric resin for scandium in the presence of titanium is not high, resulting in very poor scandium purification.
Another method for processing scandium-bearing waste from the production of aluminium, titanium, zirconium, tin, tungsten, uranium (see RU patents 1,572036; 2,196184) includes: sorption from sulphuric acid solution on an anion exchange resin (preconditioned with sulphuric acid solution); sorption of scandium by phosphorus-containing resin; desorption of scandium and processing of eluate. Sorption is carried out on weak base anion exchange resin treated with solution of sulphuric acid with concentration of 150-500 g/L. The weak base anionic resin has polyamine functionality. While this method increased the degree of scandium purification from titanium, the scandium concentration in the pregnant solution after desorption was low, as full scandium requires 4 to 5 bed volume of desorption solution per volume pregnant resin.
It is to be understood that the references to the prior art publications in this specification does by no means constitute an admission that the publications form part of the common general knowledge in the art, in Australia or any other country.